The present invention relates to a method of producing a smooth, cold-drawn, multifilament yarn from polyester POY (partially oriented yarn) and to a yarn produced by this process.
A smooth multifilament yarn is to be understood as meaning an untextured multifilament yarn which retains its uncrimped flat shape even on boiling in water.
Polyester is to be understood as meaning a thermoplastic material prepared from at least 85% by weight of terephthalic acid and ethylene glycol.
Polyester POY refers to a polyester yarn which has a linear density of about 50 to 1,200 dtex and was melt-spun at a speed of about 2,800 to 4,000 m/min.
In the present invention, cold drawing refers to drawing at yarn temperatures which are significantly below the glass transition temperature of the polyester, i.e. appreciably below 85.degree. C. For example, a delivery roller for the drawing can be unheated or be at a temperature of up to 70.degree. C. The drawing can take place with or without the use of a drawing peg.
Homogeneous cold drawing is to be understood as meaning that the draw ratio has to be chosen sufficiently high to ensure that, in the drawn yarn, there are no undrawn areas or areas drawn to less than the draw ratio, which is discernable for example in the Uster %, which in general should be &lt;1.5. For POY from the stated speed range, this means that the draw ratio shall be at least 1.6, depending on a spin speed.
For the purposes of the present invention, hot relaxation is a treatment which causes a decrease in the length of the yarn by heat treatment at temperatures above the glass transition temperature. The extent of the decrease in length is determined by the overfeed VE: ##EQU1## V.sub.L =speed of delivery system V.sub.A =speed of take-off system
The production of a cold-drawn polyester yarn from POY spun at more than 4,000 m/min is known from (JP-A-53-143,728).
DE-A-2,839,672 discloses a polyester replacement yarn for cellulose acetate, having a boil shrinkage of 2 to 6% and obtainable by direct high-speed spinning at about 4,000 m/min without the use of a drawing system or any heat treatment. In this publication, a boil shrinkage of less than 2% is referred to as extremely low and as very difficult to obtain directly.
The main disadvantage of the known yarn consists in that its shrinkage is still too high. In addition, such a yarn has to be shrunk before package dyeing and be rewound onto perforated dyeing centers.